1. Field of the Invention
The present invention relates to a method for gasifying coal in-situ and transporting the generated product gases to the earth's surface. More particularly, the invention relates to a novel and improved method for in-situ coal gasification which utilizes a radially-disposed oxidant gas or oxidant mixture injection manifold-like system embedded in the floor rock beneath the coal seam wherein the coal is ignited at the interface of the coal seam and the floor rock and product gases are recovered at production wells.
2. Description of the Prior Art
Underground coal gasification (UCG) is a conceptually simple process for the in-situ extraction of coal values. In its most general form, a pair of process wells (an injector and a producer) some specified distance apart, is drilled from the surface into the coal seam. A combustible material, e.g., charcoal, fuel oil, etc., or an electrical resistance heater, is inserted into one well to ignite the coal. Oxidant gas (usually air, or oxygen-enriched air) injected at high pressure and low volume rate is injected into the other well, permeates the coal seam, draws the flame toward the second well (by a process of reverse combustion) and in effect forms a permeable linkage between the wells. The coal seam may also be prepared and the wells linked by directional drilling, hydraulic or explosive fracturing, electrolinking, etc. Of these various methods, the method which seems least dependent on seam characteristics is directional drilling from the surface to construct a horizontal channel between the wells. Once linked, an oxidant blast (consisting of air or a mixture of steam and oxygen) is injected at high rate and low pressure into one of the process wells, and forward gasification commences, consuming the bulk of the coal between the wells and generating a mixture of combustible gases (CO, CH.sub.4, H.sub.2) and other materials (H.sub.2 O, CO.sub.2, char, coal, H.sub.2 S, tars, etc.), which exit to the surface by way of the second process well. There are disadvantages to conventional UCG operations in which the process wells only extend into the coal seam to a depth substantially above the base of the seam, or in which the method used to prepare and link the wells does not produce a reliable permeable path at the base of the coal seam, or in which the oxidant mixture is injected into the seam by piping which is a part of the process wells described above. It is well known from US-UCG experiences, in particular, that if the linkage path forms near the top of the coal, gasification reactions quickly proceed to the interface between coal and overlying roof rock. Poor sweep results, bypassing and an override condition by the injected oxidant occurs, bypassing of hot product gases occurs and devolatilization of the coal prior to gasification is inhibited, heat loss to the roof becomes significant, and a significant portion of the resource is not utilized. Additionally, and most importantly, as a result of the override conditions and the excessive temperatures produced above the bulk of the coal, the oxidant injection system can be damaged or destroyed, process well longevity drastically shortened, and excessive oxidant consumption results. The locally high temperatures may exacerbate roof collapse and promote unwanted or uncontrolled in-situ water intrustion.
Although there is prior art disclosing a radially-disposed oxidant gas or oxidant mixture injection system for in-situ coal gasification as shown in U.S. Pat. No. 3,506,309, no art exists showing such a system embedded in floor rock beneath the coal seam to be gasified. In the aforementioned patent, there is shown an injection well connected to a plurality of radially-disposed production wells. However, none of the inlet channels in this disclosure extend into the floor rock beneath the coal seam.